Aluminum sliding bearing alloy

ABSTRACT

The invention relates to an aluminum sliding bearing alloy, comprising 3 to 6 mass % zinc, 0.3 to 2.0 mass % copper, 0.2 to 1.0 mass % magnesium, 0.3 to 2.0 mass % silicon and 2 to 4.5 mass % lead. According to the invention, said alloy is obtained by means of continuous casting with a minimum dimension, i.e. a strand thickness of more than 20 mm, solidifying in a mold which is indirectly cooled only, with a withdrawal speed of 1 to 5 mm/s and with a cooling speed of less than 100 K/s.

BACKGROUND OF THE INVENTION

The invention concerns a method for manufacturing a cast productcomprising an aluminum friction bearing alloy having portions of zinc,copper, magnesium, silicon and lead, as well as the product itself. Afriction bearing alloy of this kind is e.g. Alzn4.5CuMgSiPb which theapplicant has been producing for some time under the trade name KS 961.This friction bearing alloy is distinguished by its high stability underload. It has not been possible up to now to increase the lead content toa satisfactory degree for improving the emergency running properties,i.e. to increase resistance to scuffing, since with lead contents ofmore than 1 mass %, phase separation occurs in the liquid melt in theform of a precipitation of a liquid lead phase. This separation forhigher lead content of the aluminum alloy friction bearing preventsformation of finely distributed lead precipitates. It has not beenpreviously possible to produce a superior quality friction bearingmaterial of this kind. EP 0 440 275 A1 proposes a continuous castingmethod for an aluminum alloy which can comprise one or more of thefollowing components: 1 to 50 mass % lead, 3 to 50 mass % bismuth and 15to 50 mass % indium and additionally one or more of the components: 0.1to 20 mass % silicon, 0.1 to 20 mass % tin, 0.1 to 10 mass % zinc, 0.1to 5 mass % magnesium, 0.1 to 5 mass % copper, 0.05 to 3 weights iron,0.05 to 3 mass % manganese, 0.05 to 3 mass % nickel and 0.01 to 0.3 mass% titanium, wherein the billet is chilled with direct cooling water at arate of 700 K/s. This procedure is intended to prevent formation oflarge-volume minority phase precipitates during the time period betweenarrival at the segregation temperature and solidification of the matrixmetal. It has, however, turned out that direct water cooling of thesolidifying billet is associated with large temporal and spatialfluctuations in the cooling rate, leading to inhomogeneities in the castproduct. Process stability required for series production cannot beachieved in a reproducible fashion. Moreover, due to the very highcooling rate, there is the considerable danger that cracks are formed inthe cast product.

It is therefore the underlying purpose of the present invention toimprove the emergency running properties of the above-mentioned aluminumalloy friction bearing.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention by a continuouscasting method and with a continuously cast product made from aluminumfriction bearing alloy having the features claimed.

The aluminum alloy friction bearing is preferably cast vertically

The cooling rate of less than 100 K/s is achieved in that the alloy orthe solidifying billet is not cooled by direct chilling of the billetbut by directing cooling agent onto the chilled casting mold.

In accordance with the invention, it has been determined for the firsttime that aluminum alloy friction bearings of the mentioned typecomprising an increased lead content of 1.9 to 4.5 mass %, in particularfrom 2 to 4.5, from 2 to 4, from 2.5 to 4, or from 2.5 to 3.5 mass %,can be produced using the above-mentioned processing method withsatisfactory quality with respect to the cast structure. The coolingrate of continuous casting is preferably between 20 and 50 K/s. Thewithdrawal rate of the billet is preferably between 1.5 and 2.5 mm/s.

The inventive friction bearing alloy is advantageously characterized inthat 90% of the drop-shaped lead precipitates have dimensions of lessthan 10 μm.

It has turned out that with a lead content of more than 2.5 mass %somewhat larger lead balls of a diameter of up to approximately 20 μmare sometimes produced. These have, however, no negative effects on thestrength of the friction bearing material.

Only with lead contents of approximately 3.5 mass % and more, are largerlead balls, having sizes up to a maximum of 50 μm, more frequentlyproduced. It has, however, generally turned out that with lead contentsof up to 4 mass %, and in any event, of up to 3.5 mass %, the caststructure does not show any significant stability loss.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a photomicrograph of a first aluminum friction bearingalloy manufactured with the method in accordance with the invention;

FIG. 2 shows a photomicrograph of a second aluminum friction bearingalloy manufactured with the method in accordance with the invention;

FIG. 3 shows a photomicrograph of a third aluminum friction bearingalloy manufactured with the method in accordance with the invention;

FIG. 4 shows a photomicrograph of a fourth aluminum friction bearingalloy manufactured with the method in accordance with the invention; and

FIG. 5 shows a photomicrograph of a fifth aluminum friction bearingalloy manufactured with the method in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the photomicrograph of AlZn4.5CuMgSiPb1.9. The furnacetemperature was 775° C. and the distributor temperature of thecontinuous cast system was set to 745° C. The chilled mold temperaturewas 720° C. The casting or withdrawal rate of the billet wasapproximately 2 mm/s.

The result is a perfect structure which does not differ from that of theconventional aluminum alloy friction bearing KS 961.

Similar results are obtained for the alloy according to FIG. 2 whichdiffers from the one of FIG. 1 in that the lead content is 2.5 mass %.The furnace temperature was slightly increased to 780° C. Thedistribution temperature and the chilled mold temperature remainedunchanged at 745° C. and 720° C., respectively.

FIG. 3 shows the photomicrograph of an aluminum alloy friction bearingwhich differs from the one of FIG. 1 in that it contains 3 mass % lead.The furnace temperature was 805° C., the distributor temperature was765° C. and the chilled mold temperature was 740° C. The temperatureswere increased since the segregation temperature in the phase diagramincreases with increasing lead concentration.

FIG. 4 shows the photomicrograph of a corresponding aluminum alloyfriction bearing with 3.7 mass % lead. The furnace temperature was 815°C., the distributor temperature was 775° C. and the chilled moldtemperature was 750° C.

FIG. 5 shows the photomicrograph obtained after casting of the alloyaccording to FIG. 4 which has an additional 0.2 mass % zinc and thuscontains only 3.6 mass % lead. The structure contains a larger portionof finer lead precipitates than FIG. 4. The casting parameterscorresponded to those of the above-mentioned embodiment of FIG. 4.

What is claimed is:
 1. A method for manufacturing a continuously castproduct from an aluminum friction bearing alloy, the method comprisingthe steps of: a) preparing an alloy, said alloy consisting essentiallyof 3 to 6 mass % zinc, 0.3 to 2.0 mass % copper, 0.2 to 1.0 mass %magnesium, 0.3 to 2.0 mass % silicon, 1.9 to 4.5 mass % lead, the restaluminum, unavoidable impurities and up to 0.2 mass % tin; b)introducing said alloy into an exclusively indirectly cooled chilledmold; c) continuously casting said alloy in said chilled mold with abillet thickness of more than 20 mm, a withdrawal rate of 1 to 5 mm/s,and a cooling rate of less than 100 K/s.
 2. The method of claim 1,wherein said cooling rate during continuous casting is between 20 and 50K/s.
 3. The method of claim 1, wherein said withdrawal rate duringcontinuous casting is between 1.5 to 2.5 mm/s.
 4. A continuously castaluminum friction bearing alloy consisting essentially of 3 to 6 mass %zinc, 0.3 to 2.0 mass % copper, 0.2 to 1 mass % magnesium, 0.3 to 2.0mass % silicon and 3.25 to 4.5 mass % lead, the rest aluminum,unavoidable impurities, and up to 0.2 mass % tin, wherein the leadcomprises finely distributed precipitates with 90% of drop-shaped leadprecipitates having dimensions of less than 10 μm.
 5. The continuouslycast alloy of claim 4, wherein said alloy comprises 2 to 4 mass % lead.6. The continuously alloy of claim 4, wherein said alloy comprises 2.5to 4 mass % lead.
 7. The continuously cast alloy of claim 4, whereinsaid alloy comprises 2.5 to 3.5 mass % lead.